1. Technical Field
This invention relates to the technology of catalytically converting emissions of a compressed natural gas (CNG) fueled engine, and more particularly to catalytic conversion of exhaust gases containing saturated hydrocarbons, including methane.
2. Discussion of the Prior Art
Natural gas (essentially 85% methane) is an attractive source of fuel for vehicles because it provides for a lower fuel cost, longer engine life, lower maintenance, and reduced oil consumption. Development of catalysts for high efficiency removal of saturated hydrocarbons, including methane, by oxidation within an exhaust stream is of strategic importance; it may be crucial in view of the emission control requirements promolgated by the U.S. Government. In the past, oxidation of methane has received little attention in automotive catalysis. Extreme difficulty of removal of methane is experienced because a C--H bond must be ruptured. In the oxidation of higher alkanes, oxidation is easily achieved by the cleavage of C--C bonds. Since the C--H bond is stronger, methane is more difficult to oxidize.
The prior art has investigated the use of noble metals and base metals as catalysts for stimulating the oxidation of methane by cleavage of the C--H bond. Alumina, silica, thoria, and titania supported platinum and palladium catalysts were evaluated in 1983 and 1985 (see C. F. Cullis and B. M. Willatt, Journal of Catalysis, Vol. 83, p. 267, 1983; and V. A. Drozdov, P. G. Tsyrulnikov, V. V. Popovskii, N. N. Bulgakov, E. M. Moroz, and T. G. Galeev, Reaction Kinetic Catalysis Letters, Vol. 27, p. 425, 1985). These studies showed that an alumina supported palladium catalyst is the most active, followed by an alumina supported platinum catalyst. A reduction in catalytic activity is observed when silica and titania are used as supports. A systematic study of the use of alumina supported base metal catalysts for methane oxidation was conducted in 1963; chromium was found to be the most active. At a metal loading of 3.1 weight percent, chromium was found to be comparable to palladium (see K. C. Stein, J. J. Feenan, L. J. Hofer, and R. B. Anderson, Bureau of Mines Bulletin, No. 608, U.S. Government Printing Office). However, use of only Cr.sub.2 O.sub.3 on Al.sub.2 O.sub.3 is disadvantageous because of the volatile and toxic nature of Cr.sub.2 O.sub.3 and the poor durability of the Cr.sub.2 O.sub.3 --Al.sub.2 O.sub.3 catalyst. In another article evaluating base metal catalysts for methane oxidation, unsupported Co.sub.3 O.sub.4 was found most active (see R. B. Anderson, K. C. Stein, J. J. Feenan, and L. J. Hofer, Industrial Engineering Chemistry, Vol 53, p. 809, 1961). However, use of only Co.sub.3 O.sub.4 on Al.sub.2 O.sub.3 is disadvantageous because of the volatile and toxic nature of Co.sub.3 O.sub.4 and the tendency of Co to form a low surface area spinel with Al.sub.2 O.sub.3 resulting in poor durability.
The prior art has found that the deactivation of a palladium on alumina catalyst can occur by the reaction of water vapor with palladium oxide to form Pd(OH).sub.2. It is desirable to retard the mobility of the adsorbed water vapor species and thereby reduce Pd(OH).sub.2 formation. Such prior art has also found that palladium oxide is less active than palladium, and therefore it is desirable to retain palladium in the metallic state and inhibit the formation of palladium oxide.
In the course of examining Pd on Al.sub.2 O .sub.3 at an effective three-way catalyst (converting methane, CO and NO.sub.x), the prior art has demonstrated a negative teaching to the use of lanthana with palladium (see H. Muraki, "Performance of Palladium Automotive Catalyst", SAE Technical Paper Series No. 910842, 1991). This work resulted in a conclusion that total hydrocarbon conversion of CH.sub.4, being the most difficult hydrocarbon to oxidize, by palladium/lanthanum catalysts, near stoichiometric conditions, is lower than that of a palladium catalyst by itself; La.sub.2 O.sub.3 is believed to suppress hydrocarbon oxidation activity. The prior art has also demonstrated that the use of La.sub.2 O.sub.3 with palladium increases the hydrocarbon light-off temperature (see H. Muraki et al, "Palladium-Lanthanum Catalysts for Automotive Emission Control", Ind. Eng. Chem. Prod. Res. Dev., 25 (1986) 202-208).
Lanthana has been used by the prior art with Pd/Al.sub.2 O.sub.3 catalysts in ways not related to catalyst conversion enhancement, namely, to thermally stabilize the alumina support (see U.S. Pat. No. 4,906,176). This patent teaches the use of other catalytic components, i.e., manganese, chromium, zirconium, rare earth elements, tin, zinc, copper, magnesium, barium, strontium, and calcium to promote catalytic activity. However, patent '176 fails to appreciate the conversion enhancement role La.sub.2 O.sub.3 may play during CH.sub.4 oxidation because the disclosure used the wrong and undesirable form of oxide support (i.e., La.sub.2 O.sub.3 .multidot.11Al.sub.2 O.sub.3), the lanthana was not deposited correctly, operated under generally lean conditions, and never measured the conversion efficiency attained using the above catalyst because of their primary interest in measuring thermal stability.